Semiconductor light emitting elements such as a semiconductor laser element and a light emitting diode emitting blue or violet light include gallium nitride semiconductor light emitting elements. At the time of production of a GaN-based semiconductor element, since it is difficult to produce a substrate made from GaN, a GaN semiconductor layer is epitaxially grown on a substrate made from sapphire, SiC, Si, or the like.
For example, an undoped GaN buffer layer, an n-GaN contact layer, an n-AlGaN clad layer, an n-GaN light guide layer, an InGaN multiple quantum well (MQW) active layer, and the like are sequentially formed on the face (0001) of a sapphire substrate by MOCVD (metal organic chemical vapor deposition). On the active layer, a p-GaN light guide layer, a p-AlGaN clad layer, a p-GaN contact layer, and the like are sequentially formed.
These layers from the p-GaN contact layer to a partial region of the n-GaN contact layer are removed by etching to expose the n-GaN contact layer. An n electrode is formed on the exposed upper face of the n-GaN contact layer, and a p electrode is formed on the upper face of the p-GaN contact layer.
FIG. 9 shows a diagram of a unit cell indicating plane directions of a sapphire single crystal. The crystal structure of sapphire can be approximated by a hexagonal system as in the drawing. When a GaN semiconductor layer is laminated on a sapphire substrate, the C face (0001) of a sapphire substrate is used. The GaN semiconductor laminated on the sapphire substrate in the direction (0001) has: a wurtzite crystal structure in the direction (0001); and a crystal polarity (which grows in the C axis direction) where the cation elements of Ga are in a growth surface direction as shown in FIG. 7. In other words, the layers are laminated in a direction perpendicular to the sapphire substrate, i.e., the C axis [0001] direction.
FIG. 10 illustrates band gap energies in the valence bands of the lamination region of an n-AlGaN clad layer 41, an n-GaN light guide layer 42, an MQW active layer 43, a p-GaN light guide layer 44 and a p-AlGaN clad layer 45, in a gallium nitride semiconductor emitting element produced by lamination of a GaN semiconductor layer on the above mentioned sapphire substrate. Patent Document 1: JP-A 2000-216497